1. Field of the Invention
The present invention relates to the fabrication of integrated circuits and to the fabrication of photolithographic reticles useful in the manufacture of integrated circuits.
2. Background of the Related Art
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's fabrication plants are routinely producing devices having 0.15 μm and even 0.13 μm feature sizes, and tomorrow's plants soon will be producing devices having even smaller geometries.
The increasing circuit densities have placed additional demands on processes used to fabricate semiconductor devices. For example, as circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to sub-micron dimensions, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Reliable formation of high aspect ratio features is important to the success of sub-micron technology and to the continued effort to increase circuit density and quality of individual substrates.
High aspect ratio features are conventionally formed by using photolithography to pattern a surface of a substrate to define the dimensions of the features and then etching the substrate to remove material and define the features. To form high aspect ratio features with a desired ratio of height to width, the dimensions of the features are required to be formed within certain parameters, which are typically defined as the critical dimensions of the features. Consequently, reliable formation of high aspect ratio features with desired critical dimensions requires precise patterning and subsequent etching of the substrate.
Photolithography is a technique used to form precise patterns on the substrate surface by transferring patterns from a photolithographic reticle to a substrate surface by light passing therethrough. Photolithographic reticles typically include a substrate made of an optically transparent material, such as quartz (i.e., silicon dioxide, SiO2), having an opaque light-shielding layer, or photomask, typically a metal, such as chromium, disposed on the surface of the substrate. The light-shielding layer is patterned to correspond to the features to be transferred to the substrate. Generally, conventional photolithographic reticles are fabricated by first depositing a thin metal layer on a substrate comprising an optically transparent material, such as quartz, and depositing a resist layer on the thin metal layer. The resist is then patterned using conventional laser or electron beam patterning equipment to define the critical dimensions to be transferred to the metal layer. The metal layer is then etched to remove the metal material not protected by the patterned resist; thereby exposing the underlying material and forming a patterned photomask layer. Photomask layers allow light to pass therethrough in a precise pattern onto the substrate surface.
Conventional etching processes, such as wet etching, tend to etch isotropically, which can result in an undercut phenomenon in the metal layer below the patterned resist. The undercut phenomenon can produce patterned features on the photomask that are not uniformly spaced and do not have desired straight, vertical sidewalls that are necessary to produce the critical dimensions of the features. Additionally, the isotropic etching of the features may overetch the sidewalls of features in high aspect ratios, resulting in the loss of the critical dimensions of the features. Features formed without the desired critical dimensions in the metal layer can detrimentally affect light passing therethrough and result in less than desirable patterning by the photomask in subsequent photolithographic processes.
Plasma etch processing, known as dry etch processing or dry etching, provides an alternative to wet etching and provides a more anisotropic etch than wet etching processes. In conventional dry etching processing, a plasma of etching gases is used to etch the metal layers formed on the substrate. The dry etching process has been shown to produce less undercutting and improve the retention of the critical dimensions of the photomask features with straighter sidewalls and flatter bottoms than wet etching processes.
However, it has been observed that dry etching of the reticles, which are generally square in shape, may provide variable or non-uniform etching of similar features disposed in the metal layer and located at different locations on the reticle surface. It is believed that one factor that arises in this substrate or global non-uniformity of etching features is due to the asymmetry of dry etching process chambers. For example, the location of apparatus components, such as gas inlets and pump ports have been observed to affect the uniformity of gas distribution in and exhaustion from the chamber, and coil placement has been observed to affect the uniformity of plasma formation. However, redesigning chambers is a costly and time consuming process of trial and error to remove or minimize process variations.
Therefore, there remains a need for processes for etching a photomask layer on a photolithographic reticle that produces more uniform global critical dimensions of features formed in the photomask layer.